Conventional selective call receivers (e.g., paging receivers) typically use ceramic resonator filters to generate most of the receiver's adjacent channel selectivity. Ceramic filters used in selective call receivers normally have a center frequency of 455 kHz with a design tolerance of plus-or-minus 1 kHz, and an additional plus-or-minus 1 kHz variation over temperature. However, recent advances in integrated circuit (IC) technology make it possible to integrate these filters. Integrated filters are commonly fabricated using a gyrator based implementation of a coupled resonator bandpass filter as is known to those skilled in the art. FIG. 1 illustrates a transconductance amplifier. The well known relation of the transconductance of an amplifier to its bias current is given by: EQU G=lout/Vin=1/2Vt
where:
I=the amplifier DC bias current; and PA1 Vt=KT/Q PA1 K=Boltzman's constant; PA1 T=absolute temperature; and PA1 Q=charge on an electron. From these equations, it will be apparent to those skilled in the art that the bias current I of the amplifier must be precisely generated and controlled to ensure consistent operation over a wide temperature range (-10 degrees C. to 50 degrees C.). As is known, bias current is the DC current required to set the filter to its desired frequency. Currently, bias current control is achieved by trimming components of the integrated circuit to eliminate large tolerance variations that are common in typical IC processing. Also, additional temperature compensation is necessary in conventional ICs to further restrict the variation that typically occur over the IC's operating temperature range.
where:
Thus, what is needed is a technique that eliminates the trimming requirement of wide variation component parameters over the opening temperature range of the IC.